1. Field of the Invention
The present invention relates to an aberration correcting optical unit for correcting aberration when information is recorded on or reproduced from an information recording medium such as an optical disc as well as an optical pickup apparatus having the aberration correcting optical unit and an information recording and/or reproducing apparatus (hereinafter, referred to as “an information recording/reproducing apparatus”).
2. Description of the Related Art
Optical discs such as a CD (Compact Disc) and a DVD (Digital Video Disc or Digital Versatile Disc) are known as information recording media for optical information recording or reproduction. In addition, a variety of different optical discs are now under development, such as an optical disc specialized for reproduction, a write-once optical disc capable of additionally recording information thereon, a rewritable optical disc capable of erasing information therefrom and re-recording information thereon, and so on.
Research and development is being pursued relating to high recording density optical discs and an optical pickup apparatus for the high-density discs. In addition, research and development are now under progress for a compatible pickup apparatus and information recording/reproducing apparatus which are applicable to different types of optical discs.
It is contemplated that the numerical aperture (NA) of an objective lens provided in an optical pickup apparatus is increased to irradiate an optical disc with a light beam of a smaller irradiation diameter for supporting the higher density trend of the optical disc. It is also contemplated that a short wavelength light beam is used to support the higher density trend.
An increased numerical aperture of an objective lens and the use of a short wavelength light beam, however, result in larger aberration of the light beam by the optical disc, causing difficulties in improving the accuracy of information recording and information reproduction.
For example, an incident angle range of the light beam to the optical disc becomes larger as the numerical aperture NA of an objective lens is increased, thereby resulting in a larger distribution width of the birefringence amount on the optical disc pupil plane, which is an amount depending on the incident angle. This causes a problem of a larger influence of spherical aberration resulting from the birefringence. Also, when a light beam of a short wavelength is used with an increased numerical aperture NA of an objective lens, influence of coma aberration cannot be negligible if the optical disc is inclined during recording or reproducing information so as to incline an incident angle (tilt angle) of the light beam with respect to the normal direction of the optical disc.
Further, the influence of aberration such as the above-described spherical aberration and coma aberration differs depending on the type of a particular optical disc since different types of optical discs such as CD and DVD have different structures and recording densities, thereby making it difficult to develop compatible optical pickup apparatus and information recording/reproducing apparatus.
Conventionally, an optical pickup apparatus having a liquid crystal unit for correcting aberration has been proposed for reducing the influence of the aberration as mentioned above (Laid-open Japanese Patent Application Kokai No. H10-20263).
This liquid crystal unit has a structure in which a liquid crystal element C is sandwiched between mutually opposing transparent electrodes A, B, as schematically illustrated in FIG. 1. A voltage applied between the transparent electrodes A, B is adjusted to change the alignment state of the liquid crystal element C, such that when light incident on one of the transparent electrode A (or B) passes through the liquid crystal element C, a change in birefringence is given to the light in accordance with the alignment state to emit the light to the other transparent electrode B (or A).
Further, at least one of the transparent electrodes A, B is divided into a plurality of transparent electrodes, for example, a1, a2, a3 and b1, b2, b3. Also, the transparent electrodes a1, a2, a3 are electrically isolated from one another, while the transparent electrodes b1, b2, b3 are also electrically isolated from one another.
Therefore, the liquid crystal element C can be adjusted in a plurality of different alignment states when different voltages are applied between transparent electrodes in an opposing relationship, for example, between the transparent electrodes a1, b1; between the transparent electrodes a2, b2; and between the transparent electrodes a3, b3, so that changes in birefringence in accordance with the respective alignment states can be simultaneously given to light incident thereon.
Then, the liquid crystal unit is positioned on an optical path between a light source for emitting laser light and an objective lens. The liquid crystal unit gives changes in birefringence in accordance with the plurality of alignment states to the laser light, causing the laser light to transmit therethrough to the objective lens. The objective lens converges the transmitted laser light to generate a light beam which is irradiated to an optical disc. Also, when reflected light produced by irradiating the optical disc with the light beam impinges on the liquid crystal unit through the objective lens, the reflected light is given the changes in birefringence in accordance with the plurality of alignment states, causing the reflected light to transmit, and the transmitted reflected light is detected by a photodetector. Therefore, the plurality of alignment states of the liquid crystal unit are adjusted as appropriate to reduce the influence of aberration such as spherical aberration and coma aberration.
However, gaps (SP) are provided between the respective transparent electrodes in the conventional liquid crystal unit to electrically isolate the plurality of transparent electrodes a1, a2, a3 and b1, b2, b3, as illustrated in FIG. 1. More specifically, the gaps SP are provided along the respective boundaries of the transparent electrodes a1, a2, a3, and the gaps SP are provided along the respective boundaries of the transparent electrodes b1, b2, b3.
Therefore, no voltage is applied to the gaps SP, so that the foregoing structure suffers from an inability of controlling the alignment states in the liquid crystal element C corresponding to the gaps SP. As a result, the aberration correction can be made for a light beam or reflected light passing through the transparent electrodes a1, a2, a3 and b1, b2, b3, whereas no aberration correction can be made for a light beam or reflected light passing through the gaps SP, so that a highly accurate aberration correction cannot be carried out for the light beam or the reflected light.
Also, when the transparent electrodes are divided by a larger number with the intention of making a finer correction for the influence of aberration, a large number of transparent electrodes are formed with required electrical insulating features therebetween within a limited effective optical path range in which the laser light or reflected light passes, resulting in an increased number of the gaps SP and a larger area occupied thereby. As a result, a fine aberration correction becomes difficult.
Further, different voltages applied to mutually adjoining transparent electrodes result in abrupt discontinuous alignment states produced in the liquid crystal element C corresponding to the gaps SP intervening between the transparent electrodes.